P55C (P5-MMX)
It is an improved version of the Pentium (P54C)
processor. It has doubled L1 cache (2x16 kB), MultiMedia eXtension (MMX),
and reduced power consumption. This was the first x86 processor requiring
dual voltage power supply (2.8/3.3 V). It is fabricated with Intels's
0.35 um process technology. Supported by most modern motherboards.
Available clock rates: 166MHz, 200MHz and 233MHz. The processor
performs well in average, however already there are faster processors
for less price (K6-PR2-233 and
6x86MX-PR233).

often operates in the same "jumper environment" than i486DX4!Notice:
The fastest processors that we can put into a 486 motherboard is the
Cyrix 5x86/120MHz and the
AMD 5x86/160MHz processors. They perform
comparably to the Pentium-90 MHz processor.

486DX4 (Intel)
The first Pentiums (P5) were NOT fast enough
to substantially shine out the fastest 486DX2/80MHz machines. This
was caused by the relatively SLOW system bus (VLB, or PCI), and the
write through operation of the internal cache memory. Maybe
this forced Intel to develope the i486DX4/100MHz processor, which
soon became the next standard. This processor easily outperformed
a P5-60 MHz processor equipped PC.

Cyrix 486DX
This processor is compatible with Intel's
i486DX processor, and provides
similar performance too. The inside construction of the processor
is original Cyrix design. This may cause incompatibility issues in
certain motherboards, namely the L1 cache (which is write back)
can only be used in write through mode (5% slower).
Different characteristics of the Cx486DX:

8 kByte integrated on chip L1 cache RAM, sometimes works just in write through mode